Method for manufacturing group III nitride compound semiconductor device

ABSTRACT

A method of manufacturing a group III nitride compound semiconductor device, includes providing a substrate, forming a group III nitride compound semiconductor layer having a device function, and forming an undercoat layer between the substrate and the group III nitride semiconductor layer, the undercoat layer having a surface of a peak and trough structure.

This application is a Continuation of U.S. application Ser. No.09/888,519 filed Jun. 26, 2001, which is a Continuation-in-part ofapplication Ser. No. 09/658,586 filed on Sep. 8, 2000, now abandoned,which are incorporated herein by reference.

This is a Continuation-in-part of application Ser. No. 09/658,586, filedon Sep. 8, 2000, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a group III nitride compoundsemiconductor device.

The present application is based on Japanese Patent Applications No.Hei. 11-276556, 2000-41222, 2000-191779 which are incorporated herein byreference.

2. Description of the Related Art

A group III nitride compound semiconductor device is used in alight-emitting device such as a light-emitting diode, etc. Such alight-emitting device has a configuration in which a group III nitridecompound semiconductor layer having a device function is epitaxiallygrown on a surface of a substrate, for example, formed of sapphire.

The internal stress is, however, generated between the sapphiresubstrate and the group III nitride semiconductor layer because thesapphire substrate is different in thermal expansion coefficient andlattice constant from the group III nitride compound semiconductorlayer. As a phenomenon caused by the internal stress, a bowing occurs ina growth of the group III nitride compound semiconductor on the sapphiresubstrate. If the bowing becomes too large, the crystallinity ofsemiconductor not only may be spoiled, but that the semiconductor layermay also has many cracks had inconvenience occurs in alignment ofphotolithography at the time of production of the device.

In the background art, therefore, a so-called low-temperature bufferlayer was formed between the substrate and the group III nitridecompound semiconductor layer to thereby relax the internal stress.

The growth temperature of the group III nitride compound semiconductorlayer for forming a device by a general metal organic chemical vapordeposition method (hereinafter referred to as “MOCVD” method) is 1000°C. or higher. On the other hand, the growth temperature of thelow-temperature internal stress layer is approximately in a range offrom 400 to 500° C. Hence, the temperature history of from the step ofcleaning the substrate at about 1000° C. to the growth of the group IIInitride compound semiconductor layer is high temperature (1000° C.)→lowtemperature are (400 to 500° C.)→high temperature (1000° C.). Hence, notonly was, it difficult to control the temperature but also thermalefficiency was poor.

It may be, therefore, conceived that the buffer layer is formed at ahigh temperature. The problem of bowing, however, occurs again if agroup III nitride compound semiconductor (for example, an AlN layer thesame as the low-temperature buffer layer) is grown directly on thesubstrate at a high temperature of about 1000° C.

SUMMARY OF THE INVENTION

The inventors of the present invention have made investigation over andover again to solve the problem of bowing. As a result, the inventorsthought up the present invention as follows:

A group III nitride compound semiconductor device comprises an undercoatlayer having surface on which a group III nitride compound semiconductorlayer having a device function can be formed, the surface of theundercoat layer containing inclined faces, wherein the projected arearatio of the inclined faces to the whole surface of the undercoat layeron a plane of projection is in a range of from 5 to 100%.

According to another aspect, preferably, the undercoat layer containinginclined faces is formed as a texture structure. Here, the “texturestructure” means a structure in which the surface of the undercoat layeris shaped like teeth of a saw in any sectional view, that is, acombination of a peak and a trough is repeated through an inclined face.The peaks may include those which are independent of each other aspolygonal pyramids (inclusive of cones) or those which are standing in arow like a mountain range.

In this specification, a “sectional trapezoid shape” means a shape inwhich there is a flat region at the top of each peak, and a shape inwhich the flat region is wider is referred to as a “pit shape”.

In this specification, when the projected area ratio occupied by theinclined face region on a whole plane of projection is in a range offrom 70to 100%, the shape is referred to as a “texture structure”; whenthe projected area ratio is in a range of from 30 to 70%, the shape isreferred to as a “sectional trapezoid shape”; and when the projectedarea ratio is in a range of from 5 to 30%, the shape is referred to as a“pit shape”.

Use of the aforementioned undercoat layer relaxes internal stressbetween the group III nitride compound semiconductor layer and thesubstrate including the undercoat layer. It is supposed that internalstress applied to a hetero interface is relaxed by acting in directionsparallel to the inclined faces because of the presence of the inclinedfaces in the hetero interface. If internal stress is relaxed in theaforementioned manner, the problem of bowing is reduced. As a result,not only can the group III nitride compound semiconductor layer beprevented from cracking, but also the crystallinity of the group IIInitride compound semiconductor layer can be improved. Moreover, itbecomes easy to perform alignment of photolithography at the time ofproduction of the device.

Hereupon, the undercoat layer transmits light having a wavelength of notsmaller than 360 nm because the undercoat layer is made of a group IIInitride compound semiconductor. Incidentally, when the undercoat layeris made of AlN (refractive index: 2.12) and the group III nitridecompound semiconductor layer provided on the undercoat layer is made ofGaN (refractive index: 2.60), the angle of incidence of light on theundercoat layer must be selected to be not larger than about 22 degreesso that light given from the GaN side is totally reflected on theundercoat layer. In the case of an undercoat layer having theaforementioned texture structure, it is impossible to obtain totalreflection surely on the whole surface of the undercoat layer though itmaybe said that the reflectivity of the undercoat layer is relativelyhigh because the angle of incidence of light on the surface of theundercoat layer becomes small.

From the above point of view, the present invention may be configured asfollows.

A group III nitride compound semiconductor device comprises: asubstrate; a group III nitride compound semiconductor layer having afunction light-emitting device or a function or a photodetecorlight-receiving device function; an undercoat layer formed between thesubstrate and the group II nitride compound semiconductor layer and madeof a group III nitride compound semiconductor, the undercoat layerhaving a surface formed as a texture structure or shaped like trapezoidsin section, or like pits; and a reflection layer formed on the surfaceof the undercoat layer and made of nitride of at least one kind of metalselected from the group consisting of titanium, zirconium, hafnium andtantalum, the reflection layer having a surface shape formed inaccordance with the surface shape of the undercoat layer.

According to the group III nitride compound semiconductor deviceconfigured as described above, a reflection layer made of predeterminedmetal nitride is formed on the surface of the undercoat layer having asurface shape such as a texture structure, a trapezoid shape in sectionor a pit shape. The reflection layer also has such a surface shape as atexture structure, a trapezoid shape in section or a pit shape becausethe reflection layer is formed in accordance with the surface shape ofthe undercoat layer.

The reflection layer made of metal nitride has a so-calledmetallic-color mirror surface. Moreover, the angle of incidence of lighton the surface of the texture structure, trapezoid shape in section orpit shape from the group III nitride compound semiconductor layer can bemade smaller. Hence, the reflection layer according to the presentinvention can substantially totally reflect light incident on thereflection layer from the group III nitride compound semiconductor layerside.

The inventors of the present invention have already proposed nitride ofat least one metal selected from the group consisting of titanium,zirconium, hafnium and tantalum so that a group III nitride compoundsemiconductor can be grown with good crystallinity on the metal nitridewhen the metal nitride is used as the aforementioned predetermined metalnitride (see Japanese Patent Publication No. Hei. 2000-323753). Also inthe case where a group III nitride compound semiconductor is to be grownon a reflection layer made of the aforementioned metal nitride, internalstress between the group III nitride compound semiconductor layer andthe substrate inclusive of the reflection layer and the undercoat layercan be relaxed because the surface of the reflection layer is formed asa texture structure, a trapezoid shape in section or a pit shape. Assimilar to the aforementioned, it is conceived that stress applied on ahetero interface is diffused in parallel to the inclined faces becauseof the presence of the inclined faces in the hetero interface and,accordingly, the stress can be relaxed. When internal stress is relaxedin the aforementioned manner, the bowing problem or cracking is reduced.Moreover, the crystallinity of the group III nitride compoundsemiconductor later is improved, and the group III nitride compoundsemiconductor layer can be aligned easily when the device is produced.

Features and advantages of the invention will be evident from thefollowing detailed description of the preferred embodiments described inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawings will be provided by the Patentand Trademark Office upon request and payment of the necessary fee.

FIG. 1 is a sectional view showing an undercoat layer of a texturestructure;

FIG. 2 is a surface SEM photograph showing an undercoat layer of atexture structure;

FIG. 3 is a surface SEM photograph showing an undercoat layer of asectional trapezoid shape;

FIG. 4 is a surface SEM photograph showing an undercoat layer of a pitshape;

FIG. 5 is a graph showing the quantity of bowing of a substrate;

FIG. 6 is a sectional view showing bowing of a substrate;

FIG. 7 shows a light-emitting diode according to an embodiment of thepresent invention;

FIG. 8 shows a light-emitting diode according to another embodiment ofthe present invention;

FIG. 9 is a surface SEM photograph of an undercoat layer according tothe embodiment shown in FIG. 8;

FIG. 10 is a graph for comparison between the light output intensity ofthe light-emitting diode as the embodiment shown in FIG. 8 and the lightoutput of a light-emitting diode as a comparative example;

FIG. 11 shows a light-emitting diode according to a still anotherembodiment of the present invention; and

FIG. 12 shows a light-emitting diode according to a still anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Respective constituent parts of the present invention will be describedbelow in detail.

Substrate

The material of the substrate is not particularly limited so long as anundercoat layer made of a group III nitride compound semiconductor canse formed on the substrate. Examples of the substrate material which canbe used include: hexagonal-crystal such as sapphire, SiC (siliconcarbide), GaN (gallium nitride), etc.; and cubic-crystal such as Si(silicon), GaP (gallium phosphide), GaAs (gallium arsenide), etc.

Group III Nitride Compound Semiconductor Layer

A group III nitride compound semiconductor is represented by the generalformula Al_(X)Ga_(Y)In_(1−X−Y)N (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1), which includesso-called binary compounds such as AlN, GaN and InN, so-called ternarycompounds such as Al_(X)Ga_(1−X)N, Al_(X)In_(1−X)N and Ga_(X)In_(1−X)N(0<x<1 in the above), and so-called quaternary compounds such asAl_(X)Ga_(Y)In_(1−X−Y)N (0<X<1, 0<Y<1, 0<X+Y<1). The group III elementsmay be partially replaced by boron (B), thallium (Tl), etc. The nitrogen(N) may be partially replaced by phosphorus (P), arsenic (As), antimony(Sb), bismuth (Bi), etc.

The group III nitride compound semiconductor may contain any optionaldopant. Si, Ge, Se, Te, C, etc. may be used as n-type impurities. Mg,Zn, Be, Ca, Sr, Ba, etc. may be used as p-type impurities. Incidentally,after doped with p-type impurities to reduce the resistance, the groupIII nitride compound semiconductor may be subjected to electron beamirradiation, plasma irradiation or heating by means of a furnace.

The group III nitride compound semiconductor may be formed by a metalorganic chemical vapor deposition method (MOCVD method) or any otherknown method such as a molecular beam epitaxy method (MBE method), ahalide vapor phase epitaxy method (HVPE method), a sputtering method, anion-plating method, or the like.

Examples of the device formed from the group III nitride compoundsemiconductor include optical devices such as a light-emitting diode, aphotodetector, a laser diode, a solar cell, ect.; bipolar devices suchas a rectifier, a thyristor, a transistor, etc.; unipolar devices suchas an EET, etc.; and electronic devices such as a microwave device, etc.

Incidentally, a homo structure, a single hetero structure or a doublehetero structure with an MIS junction, a PIN junction or a p-n junctionmay be used as the configuration of the light-emitting device or thephotodetector. A quantum well structure (a single or multiple quantumwell structure) may be used as the light-emitting layer and/or the clad.

The undercoat layer is not specifically limited if the aforementionedgroup III nitride compound semiconductor can be grown on the undercoatlayer.

In an embodiment, the undercoat layer is constituted by a first groupIII nitride compound semiconductor layer formed on a substrate.

Examples of the first group III nitride compound semiconductor formed onthe substrate include quaternary compound semiconductors represented byAl_(X)Ga_(Y)In_(1−X−Y)N (0<X<1, 0<Y<1, 0<X+Y<1), ternary compoundsemiconductors represented by Al_(X)Ga_(1−X)N (0<X<1), and binarycompound semiconductors by AlN, GaN and InN. Especially, AlN ispreferably used on a sapphire substrate.

Each of these first group III nitride compound semiconductor layerssubstantially has a single crystal structure.

The inclined faces are formed in the surface of the undercoat layer. Abase structure forming the inclined faces may include polygonal pyramidssuch as triangular pyramids, quadrilateral pyramids, etc., or structureslike a mountain range in which peaks and troughs are connectedalternately by belt-like inclined faces. The inclined faces are formedon the whole surface of the undercoat layer. Each of the inclined facesis so small as to have a width smaller than 2 μm on a plane ofprojection. The projected area ratio of the inclined faces to thesurface of the undercoat layer on a plane of projection is set to bepreferably in a range of from 5 to 100%, more preferably in a range offrom 30 to 100%, further preferably in a range of from 70 to 100%.

If the projected area ratio of the inclined faces on a whole plane ofprojection is in a range of from 70 to 100%, the surface of theundercoat layer exhibits a texture structure as shown in FIGS. 1 and 2and is shaped like mountains in section. If the projected area ratio is100%, the surface of the undercoat layer exhibits a structure in which acombination of a peak and a trough is repeated like teeth of a saw.

If the projected area ratio of the inclined faces on a whole plane ofprojection is in a range of from 30 to 70%, the surface of the undercoatlayer is shaped like trapezoids in section so that insular portions andmountain portions coexist as shown in FIG. 3.

If the projected area ratio of the inclined faces on a plane ofprojection is in a range of from 5 to 30%, the surface of the undercoatlayer is shaped like pits so that holes are formed in a flat surface asshown in FIG. 4.

Here, the “plane of projection” means a plane of projection obtained byperforming parallel projection of the surface of the undercoat layeronto a plane parallel with the substrate.

The first group III nitride compound semiconductor layer having a roughsurface as described above is formed by pouring a larger amount ofammonia gas (NH₃) than the general growth condition and at a hightemperature (about 1,150° C.) which is substantially the sametemperature as a second group III nitride compound semiconductor havinga device function will be formed.

Next, an experimentation for confirming the effect of the presentinvention will be described below.

An AlN layer was formed on a sapphire substrate by the MOCVD method fora reference sample. A 4 μm thick GaN layer was further formed on the AlNlayer by the MOCVD method. The quantity of bowing of each sample at roomtemperature was as shown in FIG. 5. As shown in FIG. 6, the quantity ofbowing was measured as the center height H of each sample. In FIG. 5,the symbol  indicates a result in the case where the growth temperatureand thickness of the AlN layer were 400° C. and 200 Å respectively. Thesymbol ▾ indicates a result in the case where the growth temperature andthickness of the AlN layer as a layer having a flat surface were 1,130°C. and 1.5 μm respectively. It was apparent from FIG. 5 that a largequantity of bowing occurred in the laminate when the AlN layer had aflat surface.

On the other hand, the reference symbol ◯ shows the embodiment of thepresent invention. In this case, the AlN layer had a surface of atexture structure as shown in FIGS. 1 and 2 and the growth temperatureand thickness of the AlN layer were 1,130° C. and 1.5 μm respectively.It was apparent from the result of FIG. 5 that a small quantity ofbowing equivalent to that of the background-art low-temperature bufferlayer () is shown when the AlN layer had such a texture structure.Moreover, variation in bowing became small.

Moreover, the full width at half-maximum (FWHM) of a rocking curve ofthe GaN layer formed on the AlN layer having such a surface texturestructure was 16 seconds. This value was approximately equal to that ofan n-type GaN contact layer used in an existing light-emitting device.Hence, this value exhibited sufficient crystallinity to serve as a groupIII nitride compound semiconductor layer having a device function.

Alternatively, after an undercoat layer having a flat surface is grown,the flat surface may be treated by a method such as etching, etc., tothereby shape the surface of the undercoat layer into a texturestructure, or like a sectional trapezoid shape or like a pit shape.

Preferably, a sedimentary layer may be formed between the substrate andthe undercoat layer.

When the undercoat layer is formed of a group III nitride compoundsemiconductor, the sedimentary layer may be preferably formed also of agroup III nitride compound semiconductor or of a metal nitride compoundsemiconductor. Among the group III nitride compound semiconductors,preferably Al_(X)Ga_(1−X)N (0≦X≦1), more preferably AlN may be used asthe sedimentary layer. Among the metal nitride compound semiconductors,preferably one kind or a combination of two or more kinds selected fromthe group of titanium nitride, hafnium nitride, zirconium nitride andtantalum nitride, more preferably titanium nitride may be used as thesedimentary layer. In this case, the substrate is preferably formed ofsapphire. More preferably, the sedimentary layer is formed on face a ofthe sapphire substrate.

A known method (such as an MOCVD method, a sputtering method, or thelike) for forming a group III nitride compound semiconductor or a metalnitride compound semiconductor can be used as a method for forming theaforementioned sedimentary layer.

The thickness of the sedimentary layer is not particularly limited butis set to be in a range of from several nm to hundreds nm (from tens Åto thousands Å).

According to the inventors' investigation, the interposition of thesedimentary layer between the substrate and the undercoat layer(distortion relaxing layer) makes the inclination in the surface of theundercoat layer easy to control. That is, the condition for forming asurface of a desired structure (such as a texture structure, a sectionaltrapezoid shape or a pit structure) is widened so that the surface ofthe desired structure is formed easily. Hence, a device having such anundercoat layer can be produced with good yield.

The sedimentary layer can be provided as a laminate of two or morelayers as follows.

An intermediate layer of a group III nitride compound semiconductor,preferably AlN or GaN, is formed on a first sedimentary layer which isformed on the substrate. A second sedimentary layer is formed on theintermediate layer (this operation may be repeated). The undercoat layeris formed on the second sedimentary layer.

The composition of the first sedimentary layer may be the same as ordifferent from that of the second sedimentary layer.

Also the thickness of the intermediate layer is not particularlylimited.

Reference is made to Japanese Patent Publications No. Hei.7-267796 and9-199759 as examples in which a plurality of sedimentary layers areformed.

Further, a reflection layer made of nitride may be formed on the surfaceof the undercoat layer.

As the material for forming the reflection layer, at least one membermay be selected from the group consisting of titanium nitride, hafniumnitride, zirconium nitride and tantalum nitride. Especially, titaniumnitride is preferably used. The method for growing the metal nitride isnot particularly limited but examples of the available method include:CVD (Chemical Vapor Deposition) such as plasma CVD, thermal CVD, opticalCVD, or the like; PVD (Physical Vapor Deposition) such as sputtering,reactive sputtering, laser ablation, ion plating, evaporation, or thelike; and so on.

The thickness of the reflection layer is preferably selected to be in arange of from 0.1 to 5.0 μm. If the thickness of the reflection layer islarger than the upper limit, there is a risk that the roughness of thesurface of the undercoat layer is lost so that the surface of thereflection layer is flattened. As a result, it becomes impossible toexpect relaxation of stress on the hetero interface between thereflection surface and the group III nitride compound semiconductorlayer. If the thickness is smaller than the lower limit, reflection oflight becomes insufficient. Especially, the thickness of the reflectionlayer is preferably selected to be in a range of from 0.1 to 1.0 μm.More especially, the thickness of the reflection layer is preferablyselected to be in a range of from 0.2 to 0.5 μm.

The embodiment has been described above on the assumption that a groupIII nitride compound semiconductor layer is grown on an undercoat layerand a reflection layer having inclined faces so that the group IIInitride compound semiconductor layer serves directly as a devicefunction layer. Incidentally, the group III nitride compoundsemiconductor layer may be used as an intermediate layer so that asecond undercoat layer having inclined faces for relaxing distortion canbe formed on the intermediate layer (this operation may be furtherrepeated). Hence, the internal stress of the group III nitride compoundsemiconductor layer having a device function is further relaxed tothereby improve the crystallinity thereof.

The intermediate layer may have a surface containing inclined faces (ofa texture structure, or the like) reflecting the surface structure ofthe undercoat layer or may have a flat surface.

The reflection layer is formed on the undercoat layer located in theuppermost position.

Embodiments of the present invention will be described below.

FIG. 7 shows the configuration of a light-emitting diode 10 according toan embodiment of the present invention.

Specifications of respective layers are as follows.

Layer Composition Dopant (Thickness) Transparent electrode 19 p-typelayer 18 p-GaN Mg (0.3 μm) Layer 17 Quantum well layerIn_(0.15)Ga_(0.85)N (3.5 nm) Barrier layer GaN (3.5 nm) n-type layer 16n-GaN Si   (4 μm) Undercoat layer 35 AlN (0.2 μm) Sedimentary layer 31AlN  (15 μm) Substrate 11 Sapphire (350 μm)  (face a) The number ofrepetition of quantum well and barrier layers: 1 to 10

The n-type layer 16 may be made to have a double-layered structure withan n⁻ layer of low electron density on the light-emitting layer 17 sideand an n⁺ layer of high electron density on the undercoat layer 15 side.

The layer 17 is not limited to the superlattice structure. A singlehetero structure, a double hetero structure, a homo-junction structure,or the like, may be used as the configuration of the light-emittingdevice.

A group III nitride compound semiconductor layer which is doped with anacceptor such as magnesium and which has a wide band gap may beinterposed between the layer 17 and the p-type layer 18. This is aimedat prevention of electrons imported in the light-emitting layer 17, fromdiffusing into the p-type layer 18.

The p-type layer 18 may be made to have a double-layered structure witha p⁻ layer of low hole density on the light-emitting layer 17 side and ap⁻ layer of high hole density on the electrode side. Each of the quantumwell layers may be formed of InGaAlN, including InN, GaN, InGaN andInAlN. Each of the barrier layers may be formed of InGaAlN, includingGaN, InGaN, InAlN and AlGaN, having a wider energy gap than that of thequantum well layer.

The light-emitting diode having such a configuration as described aboveis produced as follows.

First, while a hydrogen gas (H₂) is circulated into a reactor of anMOCVD apparatus, the sapphire substrate is heated to 1,300° C. tothereby clean the surface thereof.

Then, trimethylaluminum (TMA) and NH₃ are imported into the reactor atthe same substrate temperature so that an undercoat layer 15 of AlN isgrown by an MOCVD method. When the AlN undercoat layer 15 is grown to apredetermined thickness while TMA and NH₃ are imported in the conditionof 30 μmol/min and 3 SLM respectively on this occasion, the surface ofthe AlN undercoat layer 15 has a texture structure as shown in FIGS. 1and 2.

Similarly, if the flow rate of NH₃ in the aforementioned condition isreduced to a value in a range of from ½ times to ⅓ times, the surface ofthe undercoat layer 15 has a sectional trapezoid structure as shown inFIG. 3.

Similarly, if the flow rate of NH₃ in the aforementioned condition isreduced to a value in a range of from ¼ times to {fraction (1/9)} times,surface of the undercoat layer 15 has a pit structure as shown in FIG.4.

In the condition for forming a flat AlN film on the sapphire substrate,if the growth rate of AlN in a direction of a c axis (that is, in adirection perpendicular to the substrate) is compared with the growthrate of AlN in a direction perpendicular to the c axis (that is, in adirection parallel to the substrate) especially in an initial stage offormation of the AlN film, the latter speed is sufficiently larger.Hence, AlN is two-dimensionally grown in the direction parallel to thesubstrate and then three-dimensionally grown in the directionperpendicular to the substrate. That is, there is a time long enough toform a uniform growth site by migration of Al atoms and N atoms into thesurface of growth.

If the amount of N atoms on the growth surface is increased against thiscondition, the growth rate of AlN in the direction perpendicular to thesubstrate becomes higher because Al atoms are particularly bonded to Natoms in the surface of growth before appropriate migration of the Alatoms. As a result, growth in the direction parallel to the substratebecomes so ununiform that a texture structure can be produced. It may besafely said that an intermediate course of formation of the texturestructure is a sectional trapezoid structure or a pit structure.

Incidentally, if the amount of N atoms is increased more greatly, AlN,is grown not as a single crystal but as grains.

Then, while the substrate temperature is kept, an n-type layer 16 isformed and second group III nitride compound semiconductor layers 17 and18 after the n-type layer 16 are formed by an ordinary method (MOCVDmethod). In the growth method, an NH₃ and group III element alkylcompound gases such as trimethylgallium (TMG), trimethylaluminum (TMA)and trimethylindium (TMI) are supplied onto a substrate heated to asuitable temperature and are subjected to a thermal decompositionreaction to thereby make a desired crystal grow on the substrate.

Then, the p-type layer 18, the layer 17 and the n-type layer 16 arepartially removed by reactive ion etching with Ti/Ni as a mask. Thus, aportion of the n-type layer 16 on which an n-type electrode pad 21 willbe formed is exposed.

A photo resist is applied onto the semiconductor surface uniformly. Thephoto resist of the electrode-forming portion on the p-type layer 18 isremoved by photolithography. Thus, the p-type layer 18 of this portionis exposed. An Au—Co transparent electrode layer 19 is formed on theexposed portion or the p-type layer 18.

Then, a p-type electrode pad 20 and an n-type electrode pad 21 areformed by vapor deposition in the same manner as described above.

FIG. 8 shows a light-emitting diode 30 according to another embodimentof the present invention. Same parts in FIGS. 7 and 8 are referencedcorrespondingly. The description thereof will be therefore omitted.

In the light-emitting diode 30 according to this embodiment, asedimentary layer 31 of AlN is interposed between the sapphire substrate11 and the undercoat layer 35.

Specifications of respective layers are as follows.

Layer Composition Dopant (Thickness) Transparent electrode 19 p-typelayer 18 p-GaN Mg  (0.3 μm) Layer 17 Quantum well layerIn_(0.15)Ga_(0.85)N  (35 Å) Barrier layer GaN  (35 Å) n-type layer 16n-GaN Si  (4 μm) Undercoat layer 15 AlN  (1.5 μm)  Substrate 11 Sapphire(350 μm)   (face a) The number of repetition of quantum well and barrierlayers: 1 to 10

The light-emitting diode 30 having the above configuration is producedas follows.

First, reactive sputtering of an aluminum target is performed on asapphire substrate at a temperature of from 300 to 500° C. by an argongas sputtering apparatus while a nitrogen gas is imported. The sapphiresubstrate having AlN deposited thereon in the aforementioned manner isset into an MOCVD apparatus. While H₂ and NH₃ are imported, thesubstrate is heated to 1,130° C.

Then, TMA and NH₃ are imported in the condition of 30 μmol/min and 3 SLMrespectively to thereby form an AlN undercoat layer 35. The surface ofthe undercoat layer 35 has a texture structure as shown in FIG. 9 whichis a SEM photograph thereof.

The same method as shown in FIG. 7 is used as a method for forming ann-type layer 16 and layers after the n-type layer 16.

The light output of the light-emitting diode 30 formed according to thisembodiment, and the light output of a light-emitting diode (knownconfiguration) according to a comparative example in which thedistortion-relaxing undercoat layer 35 is omitted from the formerconfiguration are measured by a photo detector and compared with eachother (FIG. 10). In FIG. 10, the reference symbol ◯ shows the lightoutput of the light-emitting diode 30 according to the embodiment, andthe reference symbol  shows the light output of the light-emittingdiode according to the comparative example. It is apparent from FIG. 10that the light-emitting diode 30 of the embodiment achieves a higherlight output than that of the comparative example. It is supposed thatdistortion in the n-type layer 16, the light-emitting layer 17 and thep-type layer 18 which constitute a device structure is relaxed due tothe presence of the undercoat layer 35 to result in improvement ofcrystallinity of each layer.

FIG. 11 shows a light-emitting diode 50 according to a still anotherembodiment of the present invention. Same parts in FIG. 7 are referencedcorrespondingly. The description thereof will be therefore omitted.

In the light-emitting diode 50 according to this embodiment, areflection layer 25 of TiN with a thickness of 0.3 μm is interposedbetween the undercoat layer 15 and the n-type layer 16. Otherspecifications are same as those of the embodiment in FIG. 7.

In order to produce this light-emitting diode, after the formation ofthe undercoat layer 15, the sample is transferred to a reactor of a DCmagnetron sputtering apparatus. A DC magnetron sputtering method iscarried out to form a reflection layer 25 of TiN. Then, the sample istransferred to an MOCVD apparatus. While the substrate temperature iskept at 1130° C., an n-type layer 16 is formed. Other steps are same asa method for producing the embodiment in FIG. 7.

FIG. 12 shows a light-emitting diode 70 according to a still anotherembodiment of the present invention. Same parts in FIG. 8 are referencedcorrespondingly. The description thereof will be therefore omitted.

In the light-emitting diode 70 according to this embodiment, areflection layer 25 of TiN with a thickness of 0.3 μm is interposedbetween the undercoat layer 35 and the n-type layer 16. Otherspecifications are same as those of the embodiment in FIG. 8. Itsproducing method is same as that of the aforementioned embodiments.

Although this specification has been described while a light-emittingdevice is taken as an example, the present invention may be applied, ofcourse, to various kinds of semiconductor devices and to laminates asintermediates of these devices.

The present invention is not limited to the description of the mode forcarrying out the present invention and the description of theembodiment. It also includes various modifications that can be conceivedeasily by those skilled in the art, without departing from the scope ofclaim.

The following matters will be disclosed.

(21) A group III nitride compound semiconductor device comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer being mountain shapedin a section.

(22) A group III nitride compound semiconductor device comprising:

substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer having a surface withconcave and convex portions, wherein a projected area ratio of theconcave portions to the surface of the undercoat layer on a plane ofprojection is in a range of from 5 to 100%.

(23) A group III nitride compound semiconductor device according to theabove paragraph (22), wherein the projected area ratio of the concaveportions to the surface of the undercoat layer on a plane of protectionis in a range of from 30 to 100%.

(24) A group III nitride compound semiconductor device according to theabove paragraph (22), wherein the projected area ratio of the inclinedfaces to the surface of the undercoat layer on a plane of protection isin a range of from 70 to 100%.

(25) A group III nitride compound semiconductor device according to anyone of the above paragraphs (21) through (24), wherein the undercoatlayer is formed substantially of a single crystal.

(26) a group III nitride compound semiconductor device according to theabove paragraph (25), wherein the undercoat layer is formed of a groupIII nitride compound semiconductor and formed on a sapphire substrate.

(27) A group III nitride compound semiconductor device according to theabove paragraph (26), wherein the undercoat layer is formed of an AlNlayer.

(28) A group III nitride compound semiconductor device according to theabove paragraph (27), wherein the AlN layer has a thickness of from 0.2to 3.0 μm.

(29) A group III nitride compound semiconductor device according to theabove paragraph (27), wherein the AlN layer has a thickness of from 0.5to 1.5 μm.

(29-1) A group III nitride compound semiconductor device according toany one of the above paragraphs (21) through (29), further comprising asedimentary layer interposed between the undercoat layer and thesubstrate.

(30) A group III nitride compound semiconductor device according to anyone of the above paragraphs (21) through (26), wherein the undercoatlayer is formed of a silicon single crystal.

(41) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer having a surface of atexture structure.

(42) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer having a surface whichis trapezoid shaped in section.

(43) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer having a surface whichis pit shaped.

(44) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer having a surfacecontaining inclined faces, wherein a projected area ratio of theinclined faces to the surface of the undercoat layer on a plane ofprojection is in a range of from 5 to 100%.

(45) A laminate according to the above paragraph (44), wherein theprojected area ratio of the inclined faces to the surface of theundercoat layer on a plane of protection is in a range of from 30 to100%.

(46) A laminate according to the above paragraph (44), wherein theprojected area ratio of the inclined faces to the surface of theundercoat layer on a plane of projection is in a range of from 70 to100%.

(47) A laminate according to any one of the above paragraphs (41)through (46), wherein the undercoat layer is formed substantially of asingle crystal.

(48) A laminate according to the above paragraph (47), wherein theundercoat layer is formed of a group III nitride compound semiconductorand formed on a sapphire substrate.

(49) A laminate according to the above paragraph (48), wherein theundercoat layer is formed of an AlN layer.

(50) A laminate according to the above paragraph (49), wherein the AlNlayer has a thickness of from 0.2 to 3.0 μm.

(51) A laminate according to the above paragraph (49), wherein the AlNlayer has a thickness of from 0.5 to 1.5 μm.

(51-1) A laminate according to any one of the above paragraphs (41)through (51), further comprising a sedimentary layer interposed betweenthe undercoat layer and the substrate.

(52) A laminate according to any one of the above paragraphs (41)through (46), wherein the undercoat layer is formed of a silicon singlecrystal.

(61) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer being mountain shapedin section.

(62) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction; and

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer, the undercoat layer having a surface withconcave and convex portions, wherein a projected area ratio of theconcave portions to the surface of the undercoat layer on a plane ofprojection is in a range of from 5 to 100%.

(63) A laminate according to the above paragraph (62), wherein theprojected area ratio of the concave portions to the surface of theundercoat layer on a plane of protection is in a range of from 30 to100%.

(64) A laminate according to the above paragraph (62), wherein theprojected area ratio of the inclined faces to the surface of theundercoat layer on a plane of protection is in a range of from 70 to100%.

(65) A laminate according to any one of the above paragraphs (61)through (64), wherein the undercoat layer is formed substantially of asingle crystal.

(66) A laminate according to the above paragraph (65), wherein theundercoat layer is formed of a group III nitride compound semiconductorand formed on a sapphire substrate.

(67) A laminate according to the above paragraph (66), wherein theundercoat layer is formed of an AlN layer.

(68) A laminate according to the above paragraph (67), wherein the AlNlayer has a thickness of from 0.2 to 3.0 μm.

(69) A laminate according to the above paragraph (67), wherein the AlNlayer has a thickness of from 0.5 to 1.5 μm.

(69-1) A laminate according to any one of the above paragraphs (61)through (69), further comprising a sedimentary layer interposed betweenthe undercoat layer and the substrate.

(70) A laminate according to any one of the above paragraphs (61)through (66), wherein the undercoat layer is formed of a silicon singlecrystal.

(84) A laminate comprising:

a substrate;

a group III nitride compound semiconductor layer having a devicefunction;

an undercoat layer formed between the substrate and the group IIInitride semiconductor layer; and

a sedimentary layer formed between the undercoat layer and substrate,the undercoat layer being formed of a group III nitride compoundsemiconductor or a metal nitride compound semiconductor, the undercoatlayer having a surface of a texture structure, a surface of a sectionaltrapezoid shape, or a surface of a pit shape, the sedimentary layerbeing formed of a group III nitride compound semiconductor.

(85) A laminate according to the above paragraph (84), wherein thesedimentary layer is formed as a multilayer containing at least firstand second sedimentary layers and another group III nitride compoundsemiconductor layer is interposed between the first and secondsedimentary layers.

(86) A laminate according to the above paragraph (84) or (85), whereinthe sedimentary layer is formed of Al_(X)Ga_(1−X)N (0≦X≦1) and formed ata temperature lower than or equal to that of the undercoat layer.

(87) A laminate according to the above paragraph (84) or (85), whereinthe sedimentary layer is formed of a metal nitride compoundsemiconductor and formed at a temperature lower than or equal to that ofthe undercoat layer.

(88) A laminate according to the above paragraph (86), wherein thesedimentary layer is formed of an AlN layer.

(89) A laminate according to any one of the above paragraphs (84)through (88), wherein the substrate is formed of sapphire.

(90) A laminate according to the above paragraph (89), wherein thesedimentary layer is formed on face a of the sapphire substrate.

(100) A laminate comprising:

a substrate;

an undercoat layer formed on the substrate and made of a group IIInitride compound semiconductor, the undercoat layer having a surfaceformed as a texture structure or shaped like trapezoids in section, orlike pits;

a reflection layer formed on the surface of the undercoat layer and madeof nitride of at least one kind of metal selected from the groupconsisting of titanium, zirconium, hafnium and tantalum, the reflectionlayer having a surface shape formed in accordance with the surface shapeof the undercoat layer; and

a group III nitride compound semiconductor layer formed on thereflection layer.

(101) A laminate according to the paragraph (100), wherein thereflection layer is made of titanium nitride. (102) A laminate accordingto the paragraph (100) or (101), wherein the undercoat layer is made ofAl_(x)Ga_(1−x)N (0≦x≦1).

(103) A laminate according to the paragraph (102), wherein the undercoatlayer is made of AlN.

(104) A laminate according to the paragraph (100) or (101), wherein theundercoat layer is made of InGaAlN.

(105) A laminate according to the paragraph (100) or (101), wherein theundercoat layer is made of InAlN or InGaN.

(106) A laminate according to any one of the paragraphs (100) through(105), wherein the substrate is made of sapphire or silicon singlecrystal.

(107) A laminate according to any one of the paragraph (100) through(106), further comprising a sedimentary layer interposed between theundercoat layer and the substrate.

(108) A laminate according to the paragraph (100), wherein: thesubstrate is made of sapphire; the undercoat layer is made of AlN andhaving a surface formed as a texture structure; and the reflection layeris made of titanium nitride.

What is claimed is:
 1. A method of manufacturing a group III nitridecompound semiconductor device, the method comprising: providing asubstrate; forming a group III nitride compound semiconductor layerhaving a device function; and forming an undercoat layer between saidsubstrate and said group III nitride semiconductor layer, said undercoatlayer having a surface of a peak and trough structure.
 2. A methodaccording to claim 1, wherein said undercoat layer is formedsubstantially of a single crystal.
 3. A method according to claim 2,wherein said undercoat layer is formed of a group III nitride compoundsemiconductor and formed on a sapphire substrate.
 4. A method accordingto claim 1, wherein said undercoat layer is made of Al_(x)Ga_(1−x)N(0≦x≦1).
 5. A method according to claim 4, wherein said undercoat layeris formed of an AlN layer.
 6. A method according to claim 5, whereinsaid AlN layer has a thickness of from 0.2 to 3.0 μm.
 7. A methodaccording to claim 5, wherein said AlN layer has a thickness of from 0.5to 1.5 μm.
 8. A method according to claim 1, wherein said undercoatlayer is formed of a silicon single crystal.
 9. A method according toclaim 1, further comprising forming a sedimentary layer interposedbetween said undercoat layer and said substrate.
 10. A method accordingto claim 1, wherein said substrate is made of one of sapphire, siliconsingle crystal and silicon carbide single crystal.
 11. A methodaccording to claim 1, further comprising forming a reflection layer onsaid surface of said undercoat layer and made of nitride of at least onekind of metal selected from the group consisting of titanium, zirconium,hafnium and tantalum said reflection layer having a surface shape formedin accordance with a surface shape of said undercoat layer.
 12. A methodaccording to claim 11, wherein said reflection layer is made of titaniumnitride.
 13. A method according to claim 11, wherein said substrate ismade of sapphire, said undercoat layer is made of AlN and having asurface formed as a peak and trough structure, and said reflection layeris made of titanium nitride.
 14. A method according to claim 1, whereinthe peak and trough structure surface is formed over substantially awhole surface of the substrate.
 15. A method according to claim 1,wherein the undercoat layer is formed by metal organic chemical vapordeposition.
 16. A method of manufacturing a group III nitride compoundsemiconductor device, the method comprising: providing a substrate;forming a group III nitride compound semiconductor layer having a devicefunction; and forming an undercoat layer between said substrate and saidgroup III nitride semiconductor layer, said undercoat layer having asurface which is trapezoid shaped in section.
 17. A method according toclaim 16, wherein said undercoat layer is formed substantially of asingle crystal.
 18. A method according to claim 17, wherein saidundercoat layer is formed of a group III nitride compound semiconductorand formed on a sapphire substrate.
 19. A method according to claim 16,wherein said undercoat layer is made of Al_(x)Ga_(1−x)N (0≦x≦1).
 20. Amethod according to claim 19, wherein said undercoat layer is formed ofan AlN layer.
 21. A method according to claim 20, wherein said AlN layerhas a thickness of from 0.2 to 3.0 μm.
 22. A method according to claim20, wherein said AlN layer has a thickness of from 0.5 to 1.5 μm.
 23. Amethod according to claim 16, wherein said undercoat layer is formed ofa silicon single crystal.
 24. A method according to claim 16, furthercomprising forming a sedimentary layer interposed between said undercoatlayer and said substrate.
 25. A method according to claim 16, whereinsaid substrate is made of one of sapphire, silicon single crystal andsilicon carbide single crystal.
 26. A method according to claim 16,further comprising forming a reflection layer on said surface of saidundercoat layer and made of nitride of at least one kind of metalselected from the group consisting of titanium, zirconium, hafnium andtantalum, said reflection layer having a surface shape formed inaccordance with a surface shape of said undercoat layer.
 27. A methodaccording to claim 26, wherein said reflection layer is made of titaniumnitride.
 28. A method according to claim 26, wherein said substrate ismade of sapphire, said undercoat layer is made of AlN and having asurface formed as a peak and trough structure, and said reflection layeris made of titanium nitride.
 29. A method according to claim 16, whereinthe trapezoid shaped surface is formed over substantially a wholesurface of the substrate.
 30. A method according to claim 16, whereinthe undercoat layer is formed by metal organic chemical vapordeposition.
 31. A method of manufacturing a group III nitride compoundsemiconductor device, the method comprising: providing a substrate;forming a group III nitride compound semiconductor layer having a devicefunction; and forming an undercoat layer between said substrate and saidgroup III nitride semiconductor layer, said undercoat layer having asurface which is pit shaped.
 32. A method according to claim 31, whereinsaid undercoat layer is formed substantially of a single crystal.
 33. Amethod according to claim 32, wherein said undercoat layer is formed ofa group III nitride compound semiconductor and formed on a sapphiresubstrate.
 34. A method according to claim 31, wherein said undercoatlayer is made of Al_(x)Ga_(1−x)N (023 x≦1).
 35. A method according toclaim 34, wherein said undercoat layer is formed of an AlN layer.
 36. Amethod according to claim 35, wherein said AlN layer has a thickness offrom 0.2 to 3.0 μm.
 37. A method according to claim 35, wherein said AlNlayer has a thickness of from 0.5 to 1.5 μm.
 38. A method according toclaim 31, wherein said undercoat layer is formed of a silicon singlecrystal.
 39. A method according to claim 31, further comprising forminga sedimentary layer interposed between said undercoat layer and saidsubstrate.
 40. A method according to claim 31, wherein said substrate ismade of sapphire or silicon single crystal.
 41. A method according toclaim 31, further comprising forming a reflection layer on said surfaceof said undercoat layer and made of nitride of at least one kind ofmetal selected from the group consisting of titanium, zirconium, hafniumand tantalum, said reflection layer having a surface shape formed inaccordance with a surface shape of said undercoat layer.
 42. A methodaccording to claim 41, wherein said reflection layer is made of titaniumnitride.
 43. A method according to claim 41, wherein said substrate ismade of sapphire, said undercoat layer is made of AlN and having asurface formed as a peak and trough structure, and said reflection layeris made of titanium nitride.
 44. A method according to claim 31, whereinthe pit shaped surface is formed over substantially a whole surface ofthe substrate.
 45. A method according to claim 31, wherein the undercoatlayer is formed by metal organic chemical vapor deposition.
 46. A methodof manufacturing a group III nitride compound semiconductor devicecomprising: providing a substrate; forming a group III nitride compoundsemiconductor layer having a device function; and forming an undercoatlayer between said substrate and said group III nitride semiconductorlayer, said undercoat layer having a surface containing inclined faces,wherein a projected area ratio of said inclined faces to said surface ofsaid undercoat layer on a plane of projection is in a range of from 5 to100%.
 47. A method according to claim 46, wherein the projected arearatio of said inclined faces to said surface of said undercoat layer ona plane of protection is in a range of from 30 to 100%.
 48. A methodaccording to claim 46, wherein the projected area ratio of said inclinedfaces to said surface of said undercoat layer on a plane of protectionis in a range of from 70 to 100%.
 49. A method according to claim 46,wherein said undercoat layer is formed substantially of a singlecrystal.
 50. A method according to claim 49, wherein said undercoatlayer is formed of a group III nitride compound semiconductor and formedon a sapphire substrate.
 51. A method according to claim 46, whereinsaid undercoat layer is made of Al_(x)Ga_(1−x)N (0≦x≦1).
 52. A methodaccording to claim 51, wherein said undercoat layer is formed of an AlNlayer.
 53. A method according to claim 52, wherein said AlN layer has athickness of from 0.2 to 3.0 μm.
 54. A method according to claim 52,wherein said AlN layer has a thickness of from 0.5 to 1.5 μm.
 55. Amethod according to claim 46, wherein said undercoat layer is formed ofa silicon single crystal.
 56. A method according to claim 46, furthercomprising forming a sedimentary layer interposed between said undercoatlayer and said substrate.
 57. A method according to claim 46, whereinsaid substrate is made of one of sapphire, silicon single crystal andsilicon carbide single crystal.
 58. A method according to claim 46,further comprising forming a reflection layer on said surface of saidundercoat layer and made of nitride of at least one kind of metalselected from the group consisting of titanium, zirconium, hafnium andtantalum, said reflection layer having a surface shape formed inaccordance with a surface shape of said undercoat layer.
 59. A methodaccording to claim 58, wherein said reflection layer is made of titaniumnitride.
 60. A method according to claim 58, wherein said substrate ismade of sapphire, said undercoat layer is made of AlN and having asurface formed as a peak and trough structure, and said reflection layeris made of titanium nitride.
 61. A method according to claim 46, whereinthe undercoat layer is formed by metal organic chemical vapordeposition.
 62. A method of manufacturing a group III nitride compoundsemiconductor device, the method comprising: providing a substrate;forming a group III nitride compound semiconductor layer having a devicefunction; forming an undercoat layer between said substrate and saidgroup III nitride semiconductor layer; and forming a sedimentary layerformed between said undercoat layer and substrate, said undercoat layerbeing formed of a group III nitride compound semiconductor and havingone of a surface of a peak and trough structure, a surface of asectional trapezoid shape, and a surface of a pit shape, saidsedimentary layer being formed of one of a group III nitride compoundsemiconductor and a metal nitride compound semiconductor.
 63. A methodaccording to claim 62, wherein said sedimentary layer is formed as amultilayer containing at least first and second sedimentary layers andanother group III nitride compound semiconductor layer is interposedbetween said first and second sedimentary layers.
 64. A method accordingto claim 62, wherein said sedimentary layer is formed of Al_(X)Ga_(1−X)N(0≦X≦1) and formed at a temperature lower than or equal to that of saidundercoat layer.
 65. A method according to claim 62, wherein saidsedimentary layer is formed of a metal nitride compound semiconductorand formed at a temperature lower than or equal to that of saidundercoat layer.
 66. A method according to claim 62, wherein saidundercoat layer is made of Al_(x)Ga_(1−x)N (0≦x≦1).
 67. A methodaccording to claim 66, wherein said sedimentary layer is formed of anAlN layer.
 68. A method according to claim 67, wherein said substrate isformed of sapphire or silicon single crystal.
 69. A method according toclaim 68, wherein said sedimentary layer is formed on face a of saidsapphire substrate.
 70. A method according to claim 62, furthercomprising: forming a reflection layer on said surface of said undercoatlayer and made of nitride of at least one kind of metal selected fromthe group consisting of titanium, zirconium, hafnium and tantalum, saidreflection layer having a surface shape formed in accordance with thesurface shape of said undercoat layer.
 71. A method according to claim70, wherein said reflection layer is made of titanium nitride.
 72. Amethod according to claim 70, wherein said substrate is made ofsapphire, said undercoat layer is made of AlN and having a surfaceformed as a peak and trough structure, and said reflection layer is madeof titanium nitride.
 73. A method according to claim 62, wherein saidone of a surface of peak and trough structure, trapezoid shape, and pitshape is formed over substantially a whole surface of the substrate. 74.A method according to claim 62, wherein the undercoat layer is formed bymetal organic chemical vapor deposition.